Ok, perhaps someone can straighten me out on this. For those that are unfamiliar with the process, here’s what you need to know: The sights consist of one rear and one front sight. These sights are adjustable for different people’s eyes, and an elevation knob that moves the rear sight up and down. If you dial it out toward 800 meters, it raises the rear sight. If you dial it down toward 300m, the physical sight goes down.
When zeroing the rifle’s sights to match your eye, we shoot at a 25 meter target that’s really small. It’s the same size as a full-size target at 300 meters. The problem is that your bullet falls a good bit after flying 300 meters through the air and rises at 25 meters. Thus, making the big 300m target into a 25m target means you have to compensate for the elevation change.
Here’s where I’m confused. In order to adjust, you move the elevation knob from 300 to 325meters for zeroing and put it back to 300m when you shoot the actual 300m targets. I don’t understand this…shouldn’t that move my round higher? Here’s my logic:
- Moving the rear sight up (dialing up to 325m) means the buttstock lowers, canting the weapon up.
- A weapon pointed more toward the sky will shoot rounds more into the sky.
- Moving a target in to 25m makes me shoot it higher than if it were at 300m.
- A properly zeroed rifle would shoot straight through the center of the 25m target if aimed at the center and on 325m.
- A properly zeroed rifle would shoot straight through the center of the 300m target if aimed at the center and on 300m.
So I can’t understand why I’d compensate for rounds falling artificially high on a 25 meter target by making an adjustment that makes the rounds fall even higher and change it to make the rounds fly flatter (lower) when the target is far out and my bullet naturally hit lower.
Help. I’m so confused. Wouldn’t I want to reverse those so that a “high” condition is paired with a “low” compensation? What assumption is wrong?
Bullets are fired upwards in an arc. Any bullet fired above level passes a given height twice - once going up and once going back down. It may be that the 25m and 300m are both at that point in the M16’s ballistics.
The trajectory of the bullet is a parabola that will cross the same line at two points. Once on the way up, once on the way down. The bullet doesn’t rise out of the barrel, but the barrel is actually pointing up a very tiny bit. Its like throwing a baseball. You throw it “up” slightly to get it where you want to go.
On a M16A2 or A4 with a proper battle sight zero, this parabola crosses the center of your target at 32m and 300m. It reaches its highest point on the parabola at about 150m–roughly 7-10" above your point of aim.
On an M4 with a proper battle sight zero, this parabola crosses the center of your target at 25m and 300m–due to different muzzle velocity. This is why you dont turn your rear sight knob after zeroing an M4 like you do with an M16.
So… thinking about the bullet path and that parabola. On the M16, the bullet is still on its way up at the 25m mark. It doesn’t reach the same level as your point of aim until 32m. So it will be impacting the 25m target lower than it will be impacting the 300m target.
But when you are at the zero range, you are shooting at 25m, not 32m. You want the rounds hitting where you’re aiming. You could actually just use a target with seperate Point of Aim and Point of Impact zones like you would with an ACOG or M68 or whatever, but the Army just doesn’t do it that way. And for novice shooters, this is a good idea.
Anyway, so to compensate for the whole thing, instead of building a 32m zero range, or using different zeroing targets, we simply RAISE the point of impact at 25m when zeroing. This is done by raising a bottomed out rear site 1 click on the M16A2, and 2 clicks on the M16A4.
After zeroing, you have to remember to turn your BDC back that one or two clicks to bottom it out, or you will be hitting high on all your targets and completely missing the 150m one.
They dont impact artificially high at 25m. At 25m, your bullets are still traveling upwards and haven’t yet even crossed your point of aim (the center of your target). They dont cross the line yet until a few meters later. So they are low and you want to raise them to match.